1. Field of the Invention
This invention relates to a semiconductor device which is similar in construction to a lateral bipolar transistor and performs functions similar to those of the latter.
2. Description of the Prior Art
There have heretofore been proposed bipolar and MIS transistors. The bipolar transistors are classified into those of lateral construction and those of vertical construction.
In general, the construction of the lateral bipolar transistor is such that a semiconductor substrate serving as a base region has formed therein as by diffusion from the upper surface thereof an emitter and a collector region which are opposite in conductivity type to the substrate and that the semiconductor substrate serving as the base region and the emitter and collector regions have deposited thereon base, emitter and collector electrodes, respectively. On the other hand, the construction of the vertical bipolar transistor is generally such that a semiconductor substrate serving as a collector region has formed therein as by diffusion from the upper surface thereof a base region opposite in conductivity type to the substrate, that the base region has formed therein as by diffusion from the upper surface thereof an emitter region opposite in conductivity type to the base region and that the semiconductor substrate serving as the collector region and the base and emitter regions have deposited thereon collector, base and emitter electrodes, respectively.
Further, the MIS transistor is usually composed of a semiconductor substrate of one conductivity type, source and drain regions opposite in conductivity type to the substrate and formed in the substrate as by diffusion from the upper surface of the substrate, an insulating layer deposited on the semiconductor substrate at least between the source and drain regions, and gate, source and drain electrodes deposited on the insulating layer and the source and drain regions, respectively.
Thus, whether it is lateral or vertical, the bipolar transistor has such an advantage that it can be produced easily as compared with the MIS transistor because its manufacture does not involve the step for the formation of the thin insulating layer on the gate electrode which is necessary for the MIS transistor. Moreover, in the lateral bipolar transistor, the emitter and collector regions can be formed simultaneously in the semiconductor substrate serving as the base region, so that the lateral bipolar transistor can be produced with more ease than the vertical bipolar transistor that the emitter region is formed in the base region after the formation of the latter in the semiconductor substrate serving as the collector region.
Further, since the base and emitter regions of the lateral bipolar transistor has the photodiode construction, the lateral bipolar transistor has the function of a photoelectric conversion element in addition to the functions of an amplifying element and a switching element, as is the case with the MIS transistor. Accordingly, by forming many lateral bipolar transistors on a common semiconductor substrate in the so-called monolithic manner, a large-scale logic circuit or a sensor of image pickup apparatus can be constructed.
However, the conventional lateral bipolar transistor has such a construction that minority carriers injected from the emitter region into the base region in the operative state of the transistor unnecessarily spread into other parts than that of the base region defined between the emitter and collector regions. This introduces such a defect that, for example, in the case of forming a plurality of lateral bipolar transistors on a common semiconductor substrate, the operation of each transistor affects those of the others.
Accordingly, in the case of forming a monolithic logic circuit or a sensor of image pickup apparatus with a plurality of conventional lateral bipolar transistors, it is necessary to provide means for electrically isolating them from one another. This inevitably increases the number of manufacturing steps of the integrated logic circuit or the sensor of image pickup apparatus and makes it bulky.